This invention relates to an apparatus for measuring the pre-tension of a threaded bolt serving particularly for closing a reactor pressure vessel. The measurement is accomplished by means of a part which is introduced into a central drill hole of the threaded bolt and which is fastened at the lowest point of the hole, eg. where it is closed by the head of the threaded bolt, and by means of a length measuring device which determines the length difference between the threaded bolt and the introduced part.
Such a measuring arrangment is shown in U.S. Pat. No. 3,877,326 in connection with the threaded bolts used for closing a nuclear reactor vessel. There, a rod is introduced into the central hole of a bolt and is screwed in the end of the hole in the head of the threaded bolt. When the threaded bolt is stretched to obtain pre-tension, the rod in the hole remains unstressed and the pre-tension of the threaded bolt is determined by the difference in length between the inserted rod and the threaded bolt, as measured by a length measuring device such as a dial indicator.
Since, in this prior art measuring apparatus, the rod used for the measurement must be inserted and screwed fast into the hole of each threaded bolt and then removed after the measurement so as to leave the hole free for other non-destructive material tests which are part of normal maintenance, much time is consumed in the performance of this part of the test routine. As the lengthening of the threaded bolt required for the pre-tensioning is usually accomplished by mechanical and, usually, automatic tensioning devices, it is an object of the invention to provide a similar automatic measuring apparatus which can be attached to the tension device.